The treatment of cancer has progressed significantly with the development of pharmaceuticals that more efficiently target and kill cancer cells. To this end, researchers have taken advantage of cell-surface receptors and antigens selectively expressed by cancer cells to develop drugs based on antibodies that bind the tumor-specific or tumor-associated antigens. In this regard, cytotoxic molecules such as bacteria and plant toxins, radionuclides, and certain chemotherapeutic drugs have been chemically linked to monoclonal antibodies that bind tumor-specific or tumor-associated cell surface antigens (see, e.g., International Patent Applications WO 00/02587, WO 02/060955, and WO 02/092127, U.S. Pat. Nos. 5,475,092, 6,340,701, and 6,171,586, U.S. Patent Application Publication No. 2003/0004210 A1, and Ghetie et al., J. Immunol. Methods, 112: 267-277 (1988)). Such compounds are typically referred to as toxin, radionuclide, and drug “conjugates,” respectively. Often they also are referred to as immunoconjugates, radioimmunoconjugates, and immunotoxins. Tumor cell killing occurs upon binding of the drug conjugate to a tumor cell and release or/and activation of the cytotoxic activity of the drug. The selectivity afforded by drug conjugates minimizes toxicity to normal cells, thereby enhancing tolerability of the drug in the patient.
Processes for conjugating antibodies to sulfhydryl-containing cytotoxic agents such as maytansinoids have been described previously (see, e.g., U.S. Pat. Nos. 5,208,020, 5,416,064, and 6,441,163). For example, U.S. Pat. Nos. 5,208,020 and 5,416,064 disclose a process for manufacturing antibody-maytansinoid conjugates wherein the antibody is first modified with a heterobifunctional reagent such as described in U.S. Pat. Nos. 4,149,003, 4,563,304 and U.S. Patent Application Publication No. 2004/0241174 A1. U.S. Pat. Nos. 5,208,020 and 5,416,064 further describe conjugation of a modified antibody with an excess of a sulflhydryl-containing cytotoxic agent at pH 7, followed by purification on Sephadex™ G25 chromatography columns. Purification of antibody-drug conjugates by size exclusion chromatography (SEC) also has been described (see, e.g., Liu et al., Proc. Natl. Acad. Sci. (USA), 93: 8618-8623 (1996), and Chari et al., Cancer Research, 52: 127-131 (1992)).
The processes that have been previously described for manufacture of the antibody-drug conjugates are complex because they are encumbered with steps that are cumbersome to perform or produce immunoconjugates that are less pure or less stable than optimally desired. For example, conjugation at a pH of between 6.0 and 6.5 is not optimal for producing pure and stable conjugates. In addition, the conjugation reactions under these conditions are generally slow and inefficient, leading to a requirement for excessive time and material usage.
It would be desirable to modify or eliminate one or more manufacturing steps without compromising product quality, such as purity and/or stability. It would be further desirable to have additional purification options than those that have been so far described inasmuch as some options will be more efficacious with certain combinations of cell binding agents, linkers, and drugs, than with others.
In view of the foregoing, there is a need in the art to develop improved methods of preparing cell-binding agent-drug conjugate compositions that are of substantially high purity and at the same time have greater stability. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.